The field of the invention is piezoresistive pressure sensors.
Prior art silicon piezoresistive pressure sensors are made by etching thin diaphragms into the monocrystalline silicon. Piezoresistors are implanted into the silicon and because of the large piezoresistive effect in the silicon a good pressure sensor results. Such sensors have proven to be excellent low cost, high performance devices and are completely satisfactory where operating temperatures remain no higher than about 150.degree. C. One example of the prior art is the disclosure of U.S. Pat. No. 4,054,497.
In certain applications of piezoresistive sensors operating temperatures will be in the range above 150.degree. C., for example up to about 200.degree.-250.degree. C. The silicon device cannot operate at these elevated temperatures because the transducer performance degrades due to the limitations caused by the approximately 1 eV bandgap of silicon.
In the present invention to achieve higher temperatures, an epitaxial layer of GaAs is grown on a single crystal silicon structure and resistors are implanted into the GaAs layer. In this way the excellent mechanical properties of silicon as well as the chemical inertness of the Si/SiO2 surface is retained on the inside of the diaphragm while the piezoresistive sensing elements are formed in the larger bandgap semiconductor layer on the reverse side of the diaphragm which allow higher temperature operation.